I had a quick look at a document sent out to soldiers on German military abbreviations as I though it looked more like "PA RMS" considering how every other letter is positioned, I had a look for "PA", "RMS" "RNS" "PARMS" "Pa.Rns" and a bunch of others. The only one that yielded any results was "PA". The possibilities are:
P.A - Panzerabwehr - Antitank defence.
PA Personalamt - Personnel Office
PA Propagandaamit - Propaganda office
The top and the bottom make the most sense, the top far more so. Artillery was the primary antitank measure and the round is a 77mm as you stated, exactly what was used. RMS or RNS gets no results so 19-19 is a more interesting figure, there is a 19th division of the German Empire and the "Artillerie-Kommandeur 19
" is added to this late into WW1. I don't understand how the numerical system works and doesn't help that I don't speak German but the 19-19 could be referring to this. The RMS/RNS could be a person or a unit within this or something similar, there is a start and somebody who is more knowledgeable on the subject could point you in the direction better than I can as I only had a quick 20 min search and no knowledge at my disposal.